A typical electric power system is a commercial electric power system supplied by an electric power company, and the power supply quality of the commercial electric power system is maintained mainly by the electric power company.
On the other hand, the power supply quality of a stand-alone power supply system not connected to the commercial electric power system needs to be maintain by itself. Examples of the stand-alone power supply system include a power supply system in a ship and a micro grid that is a network formed by combining plural types of power supplies in a certain region.
PTL 1 discloses a power supply system in a ship, the power supply system including a plurality of auxiliary power generators, an electric power storage device configured to perform charge and discharge, and an electric propulsion unit. The power supply system further includes: an electric power detecting unit configured to detect electric power consumed by the electric propulsion unit; a start-up switch configured to output a start-up command; and a control circuit configured to change a mode of the electric power storage device into a charge mode when a detection signal of the electric power detecting unit is equal to or lower than a threshold value and change the mode of the electric power storage device into a discharge mode by the start-up command of the start-up switch to cause the electric power storage device to discharge the stored electric power to a ship electric power bus line. In a case where the ship comes into a port and sails while lowering a propulsion power of the electric propulsion unit, and the power consumption of the ship becomes equal to or lower than a threshold value, the control circuit causes the auxiliary power generators to charge the electric power storage device. In a case where the control circuit receives the start-up command of the start-up switch during the above charge operation, the control circuit causes the electric power storage device to discharge the electric power to supply the discharged electric power to the electric propulsion unit. Then, in a period from when the ship comes into a port until when the ship comes alongside a pier, an auxiliary prime mover configured to drive the auxiliary power generator is efficiently operated. With this, the amount of fuel used by the auxiliary prime mover is reduced, and an exhaust gas is also reduced, which are disclosed in PTL 1.
Such a power supply system in the ship is typically a stand-alone power supply system that is independent from the commercial electric power system. While the ship is moored at the pier, the power supply system in the ship is connected to the commercial electric power system to receive electric power.
PTL 2 discloses a technology in which in a micro grid including a prime mover power generator, a distributed power supply utilizing natural energy, such as solar power generation and wind power generation, and a power storage facility having a secondary battery, the stability and quality of the micro grid are secured by using an electric power converter included in the power storage facility.
The power supply system in the ship, the micro grid, or the like is a combined power generation system including a prime mover power generator, an electric power storage device constituted by a secondary battery and an electric power converting unit, and various power supplies, such as a power generator using a solar battery and a power generator using a fuel cell.
PTL 3 discloses a technology regarding an electric power converter used in an electric power storage device and capable of covering a harmonic component and unbalanced component of a load current without additionally providing a detecting unit configured to detect, for example, a load current. To be specific, the electric power converter includes a control portion and an electric power converting portion capable of converting DC power of a secondary battery into AC power and converting AC power, input through an output line, into DC power to store the DC power in the secondary battery. A virtual electric power generator is provided in advance instead of the electric power converting portion and the secondary battery in the control portion, and the control portion includes: a virtual electric power generator model portion configured to calculate a current value to be output, based on the voltage of the output line of the electric power converting portion and set the obtained current value as a current command value; and a control signal generating portion configured to output to the output line a current corresponding to the current command value. Then, in the virtual electric power generator model portion, an engine model converts a fuel supply amount, calculated by a governor model, into a mechanical torque of an engine without considering a response characteristic of the engine, to calculate an angular velocity and phase angle of the power generator by using the mechanical torque, which is disclosed in PTL 3.
There are two types of electric power converters that are a current control type electric power converter and a voltage control type electric power converter depending on control methods. PTL 4 discloses a technology regarding a current control type electric power converting unit. To be specific, PTL 4 discloses a technology regarding a parallel operation device configured to suppress a load change of a stand-alone power supply system constituted by an electric power storage device using a DC power supply for electric power storage and a synchronous power generator. Specifically, PTL 4 discloses a technology regarding the parallel operation device configured such that to adequately respond to a sudden load change or unbalanced load in the parallel operation device, an active current set value is obtained by inputting to a frequency control unit a signal generated by applying to a frequency set value a negative correction amount that is based on drooping characteristics of the frequency of the stand-alone power supply system, and a reactive current set value is obtained by inputting to a voltage control unit a signal generated by applying a negative correction amount that is based on drooping characteristics of the voltage to a difference between a voltage set value and a voltage effective value of the stand-alone power supply system.